Depredation rates and predators at artificial bird nests in an unfragmented northern hardwoods forest

Spatial and temporal patterns of predation on birds' nests must be understood to determine their effects on bird populations. We used artificial bird nests containing Japanese quail (Coturnix japonica) eggs to examine effects of forest strata (ground, shrub, subcanopy), time of season (early vs. midsummer), and habitat (deciduous vs. mixed coniferous-deciduous) on nest depredation rates in an unfragmented and relatively undisturbed temperate forest in the White Mountains, New Hampshire. We found that depredation rates differed significantly and most strongly among forest strata (P < 0.002), with nests on the ground having the lowest rate of depredation (31-46%) and those in the shrub layer having the highest (73-88%). Seasonal and habitat variation were also evident, but no consistent trends were found between early and midsummer or between habitats. Depredation rates at artificial nests in shrub and subcanopy strata were 2-3 times higher than depredation of real birds' nests in those same strata in the same years. Using cameras at a separate set of artificial nests on each plot, we identified 7 predator species, of which the red squirrel (Tamiasciurus hudsonicus) was the most frequent in 1990 and the fisher (Martes pennanti) in 1991. Statistical and photographic evidence suggested that fisher in 1991 located and depredated artificial nests systematically possibly by following human scent. These results identify the importance of nest strata, season, and composition of the predator community for the nesting success of forest songbirds in large tracts of undisturbed forest, and emphasize several important potential biases in studies using artificial nests.